1. Field of the Invention
This invention relates to a semiconductor acceleration sensor and to a vehicle control system using the semiconductor acceleration sensor.
2. Description of the Related Art
A known semiconductor acceleration sensor, particularly of a semiconductor electrostatic capacitance type and a semiconductor electrostatic servo type, is described in Japanese Patent Laid-Open No. 62-27666 (1987) and EP-A-0338688. In the reference a cantilever which forms an inertia body has a conductive movable electrode of predetermined mass at one end and the movable electrode is disposed between opposed fixed conductive electrodes which are stationary with respect to the movable electrode, there being a gap between the movable electrode and the fixed electrodes.
The inertia force on the movable electrode and the electrostatic force between the movable and fixed electrodes are arranged to balance one another and the position of the movable electrode is controlled to be at a fixed position independent of the acceleration by changing the electrostatic force. The electrostatic force required is dependent on the gap between the movable and fixed electrodes and decreases with increasing gap width.
In the prior art reference, the gap between the movable electrode and the fixed electrodes is required to be as wide as possible since, otherwise, there is the problem that during acceleration, or deceleration, the conductive surfaces of the movable electrode come into contact with the opposing electrode surface of the corresponding fixed electrode. As a result, due to current passing through the electrodes, a short circuit may occur resulting in a high current flowing through the small area of contact between the electrodes which, in turn, results in melting the conductive material of the electrodes and fusing the electrodes together. It will be realized that, once fused together, the sensor is useless.
Another difficulty is that the static charges that accumulate on a human body during the sensor manufacturing process may transfer to the sensor, generating a high voltage between the inertia body surface and the electrode surfaces. The force of electrostatic charge attracts the inertia body surface and the electrode surfaces to each other and may cause them to come into contact with each other. This in turn results in a short-circuit current flowing, fusing the movable electrode to one of the fixed electrodes. Such a fusing difficulty keeps the sensor yield at low levels and is a main factor contributing to the high cost of the sensor.
A possible countermeasure would be to increase the air gap between the inertia body and the stationary electrodes. However, such a step degrades the sensor's detection sensitivity and accuracy.
Another problem is that, without a desired damping, the inertia body will oscillate with large amplitudes at a resonance frequency. Such oscillation limits the measuring range of acceleration, making a highly precise measurement impossible.
An object of a first aspect of this invention is to provide a semiconductor acceleration sensor, for example of semiconductor electrostatic capacitance type or semiconductor electrostatic servo type, which can prevent malfunctions caused by the above-mentioned electrode fusing and which also has the ability to generate an oscillation damping force. An object of a feature of the first aspect of this invention is to realize a vehicle control system that can be incorporated into the small space available in vehicles and which can still measure and control the vehicle with high sensitivity and precision.
A second aspect of this invention will now be described relating to a further difficulty associated with the sensor described in the forementioned Japanese Patent Laid-Open No. 62-27666 (1987). In FIGS. 2 to 5 of the reference the chip structure of the acceleration sensor is shown. However, the reference does not refer to the reliability and cost of the heat resistance of the material used for the fixed electrodes, the adhesive strength between the fixed electrodes and substrates, the effects of the thickness of the upper and lower substrates on the characteristics, the shock resistance of the cantilever, and the management of the gaps between the movable electrode and fixed electrodes from the viewpoint of ease of assembly. In this respect, practical embodiments require consideration of the material used and structure of the sensor to achieve ease of assembly.
In the prior art reference measures for preventing thermal or electrical damage caused during the machining process of the detector have not been considered, imposing problems such as degradation of the fixed electrodes or of the substrate supporting the fixed electrodes. To assemble a silicon member forming the cantilever and movable electrode, and a borosilicate glass (for example, Pyrex.RTM. glass #7740) substrate containing an alkaline component forming the fixed electrodes, and to produce gaps with a predetermined width between the fixed electrodes and the movable electrode, the said silicon member and said substrate are anodically bonded and laminated. In such assembly, the members are heated to a temperature between 300.degree. C. and 400.degree. C. at a high voltage ranging from 200 V to 1000 V, which causes degradation to the fixed electrodes or dielectric breakdown of the Pyrex.RTM. glass. In this respect, small mounds appear in the electrode surface which are, typically, 3 .mu.m or more in amplitude. Since the gap between the fixed electrode and movable electrodes is required to be only about 3 .mu.m, the movable electrode can, therefore, not move, as required for inertia detection.
Furthermore, if there is a great difference in thickness between the upper and lower substrate supporting the fixed electrodes, the displacement caused by thermal deformation is different between the upper and lower substrates so that the electrostatic capacitance changes and the temperature characteristics thereof are worsened.
Also, if the ratio of the weight of the movable electrode to that of the cantilever is not appropriately defined, it is difficult to simultaneously satisfy both the detection accuracy and the shock resistance required of the sensor. The gap width between the movable electrode and each of the fixed electrodes will greatly affect the cost from the viewpoint of detection accuracy, machinability and assembly.
Accordingly, it is an object of a second aspect of this invention to at least partially mitigate the above enumerated problems.
It is known from "Silicon Microaccelerometer" transducers 1987 at pages 395 to 398 in the fourth International Conference on Solid-State Sensors and Actuators, June 1987, that an electrostatic servo circuit may be used for driving an electrostatic capacitor sensor. As also indicated in Japanese Patent Laid-Open No. 1-253657 (1989), a capacitance-type sensor using a pulse width modulation electrostatic servo circuit has been proposed.
It is generally known that a sensor varies in sensitivity and the zero point thereof depends on various factors during manufacture. Therefore, it is necessary to adjust the sensitivity at zero point in some manner. For example, the resistance may be changed by using a variable resistor or the printed resistor on an alumina substrate may be trimmed by using a laser, as is present day common practice, although the electronic parts and space required on the printed circuit board for such adjustment result in a larger and more expensive sensor.
It is an object of a second feature of this invention to provide a semiconductor acceleration sensor which is less expensive to produce and yet which is provided with an output adjustment circuit for accurately adjusting the sensitivity and zero point, and wherein a more compact electronic circuit is achieved than hitherto.
In Japanese Patent Laid-Open No. 64-25062 (1989) there is disclosed an acceleration detector formed by a magnet slidingly disposed in a housing, the magnet having spaced coils thereabout, whereby movement of the magnet by acceleration causes a differential induced EMF in the coils, and the unit is mounted on a printed circuit board with a control circuit therefor. In the prior art reference, however, no consideration is given to the reliability of the detector unit and control circuit against changes in environmental conditions, for example, temperature and humidity. Insufficient consideration has also been given to external radio frequency waves caused, for example, by the ignition device of the vehicle which tends to cause detector failures in the prior art device.
It is, accordingly, an object of a third feature of this invention to make a semiconductor acceleration sensor which is compact and light so that it can be mouted at any location in the body of an automobile, to ensure reliability such that the sensor can withstand changes in temperature, humidity and corrosive gases that exist in an engine compartment, and to ensure that the sensor is shielded from the effects of external radio waves.